Various systems have already been described for adapting the stiffness and/or the hardness of damping in a vehicle suspension.
U.S. Pat. No. 4,730,816 describes an apparatus for controlling the hardness of a suspension for a road vehicle; the apparatus has two chambers interconnected by a pipe fitted with an electromagnetically controlled valve having two constrictions, and a hydropneumatic accumulator acting as a spring; the single valve is fast, and it influences the hardness both of the spring and of the damper as a function of signals delivered by sensors to an electronic apparatus that controls the position of the valve. The hardness of damping is measured at all times and its desired value is calculated by the electronic apparatus, which value is then applied to the control valve; the hardness of damping and of the suspension can vary as a function of the state of the road, the state of the load, the acceleration of the vehicle, or the speed of the vehicle, e.g. in application of a servo-control relationship that reduces the hardness of the suspension at slow speed.
U.S. Pat. No. 5,276,622 describes a system for controlling variation in the damping coefficient of a damper for the purpose of avoiding end-of-stroke knocks.
U.S. Pat. No. 6,120,009 describes a shock absorber for an aircraft landing gear of length, stiffness, and damping hardness that are variable as a function of signals delivered by sensors to an electronic apparatus controlling three valves that control the flow rate of fluid entering or leaving one of the chambers of the shock absorber; the sensors may be sensitive to operating parameters of the damper such as the speed of its piston, to operating parameters of the airplane such as its speed, and/or to parameters of the landing gear.
The document “Testing of semiactive landing gear control for general aviation aircraft” by Gian Luca Ghiringhelli, Journal of Aircraft, Vol. 37, No. 4, July-August 2000, pp. 606-616, proposes mathematical models for the behavior of landing gear tires and dampers, and describes experiments for validating those models and their applications to controlling a valve for controlling the hardness of a damper.
U.S. Pat. No. 6,474,598 describes an oleopneumatic damper for landing gear including an electromagnetic coil for controlling the viscosity of a magneto-rheological oil flowing from one chamber of the damper to the other.